1. FIELD OF THE INVENTION
This invention generally relates to photoelectric control devices which detect changes in the intensity of light and produce electric signals in response to changes in the intensity of light, more particularly an improved photoelectric control device provided with a photoelectric sensing unit to cooperate with a silicon-controlled rectifier and a bridge rectifier to cause a triac to become conductive or non-conductive, so as to turn on or turn off a system when the intensity of light changes from or reaches a predetermined level.
2. BRIEF DESCRIPTION OF PRIOR ART
Photoelectric control devices have been widely used in controlling various types of equipment such as automatic lighting systems. A conventional photoelectric control device PS1, as shown in FIG. 1, includes a photoelectric sensing unit P1, a switch SW1, and a load Wl to which electric power is supplied through switch SWl, wherein switch SW1 is operable with a bimetal M in association with a heater H through which photoelectric sensing unit P1 is connected to a power source E. In operation, electric current is supplied to heater H which thus generates heat to cause bimetal M of switch SW1 to warp to cause the pair of contact points S to open or close, or to cause switch SWl to turn on or to turn off. The electrical resistance of photoelectric sensing unit P1 varies with the intensity of the light received by photoelectric sensing unit P1 to cause the electric current passing through photoelectric sensing unit P1 to vary to operate switch SW1 accordingly. In this arrangement, at a certain point during the operation of switch SW1 the pair of contact points S may pause or hesitate in between the "open" position and the "closed" position when the intensity of the light changes at a relatively slow rate. Such a pausing or hesitation of the contact points S in an interim position will cause the device to function improperly. Furthermore, the operation of switch SW1 in response to changes in the intensity of light is relatively slow and inaccurate; thus its application is very limited.
Another conventional photoelectric control device PS2, as shown in FIG. 2, includes a photoelectric sensing element P2 having a capacitor C1 shunted across two ends or junctions A and B of photoelectric sensing element P2, a resistor R1 having one end connected to junction B and another end connected to terminal C; a triac T1 having a second anode connected to terminal C, a first anode connected to junction A, and a gate connected to junction B with bilateral diode thyristor D1. The photoelectric control device PS2 is connected with a load W2 in series and then connected to a power source E. In operation, the voltage at junction B varies as the intensity of the light received by photoelectric sensing element P2 changes; when the voltage at junction B decreases, for example, to a certain level, bilateral diode thyristor D1 is activated to trigger triac T1, thus causing triac T1 to become electrically conductive, so as to "turn on" load W2. On the other hand, when the voltage at junction B decreases to a certain level, bilateral diode thyristor D1 acts to "turn off" triac T1, thus "turning off" load W2. In this arrangement, the response time of the device is considerably shortened in comparison with first conventional photoelectric control device PS1 of 10 FIG. 1; however, the switching point tends to shift or vary from the preset light condition, making the switching function inaccurate.